The present invention relates to an Fexe2x80x94Ni alloy having high strength and a small average thermal expansion coefficient. More particularly, it relates to an Fexe2x80x94Ni alloy which is applied to a shadow mask, a Braun tube (i.e. cathode-ray tube) with the shadow mask, a lead frame, a semiconductor element with the lead frame, components of a precision machine and so on.
Fexe2x80x94Ni alloys of the Invar alloy system, of which typical example is an Fexe2x80x9436% Ni alloy, and Fexe2x80x94Nixe2x80x94Co alloys of the super Invar alloy system, of which typical example is an Fexe2x80x9431% Nixe2x80x945% Co have been well known as low thermal expansion alloys each having a small thermal expansion coefficient. The Fexe2x80x94Ni alloys and Fexe2x80x94Nixe2x80x94Co alloys have been utilized for the uses which require low thermal expansion property and which are, for example, shadow masks in displays of televisions and computers, electron gun electrodes of Braun tube, or lead frames used typically for semiconductor packages.
For example, mild steel has been conventionally used as the material of the shadow masks. Although mild steel has good press-deformability and etching property, it has a large thermal expansion coefficient of about 12xc3x9710xe2x88x926/xc2x0 C., so that it has a problem that a thermal expansion of mild steel by heating with electron beam irradiation causes deterioration of color purity. Recently, while there has been an increasing demand for displays with high resolution and high brightness, the Invar system alloys each having a small thermal expansion coefficient, typically the Fexe2x80x9436% Ni alloy, have been put into practical use as an alternative shadow mask material to conventional mild steel. However, the Fexe2x80x9436% Ni alloy of the Invar system has problems that it has inferior etching property than mild steel and is very expensive.
Furthermore, under a recent rapid progress of displays toward a flat-face configuration, a tensioning system, in which a shadow mask is supported by a mask frame under a tension, has been increasingly utilized instead of the conventional shadow mask produced by press forming. In the tensioning system, since the shadow mask is supported by the frame, it is not required to have particular strength for keeping its shape. Thus, a development of a thinner shadow mask is in progress for a cost reduction purpose. However, with regard to the handling of such a thin sheet, a handling property (i.e. strength) is required therefor so as to be able to bear bending and deformation at each handling step.
With regard to the lead frame, since it is bonded to a semiconductor element having a small thermal expansion coefficient, an Fexe2x80x94Ni alloy, of which typical example is an Fexe2x80x9442% Ni alloy having a thermal expansion coefficient close to that of the semiconductor element, is used for the lead frame material. The lead frame material is subjected to fine lead working of press-punching or photo-etching. Since there is an increasing demand for finely worked shapes of high precision as semiconductor devices become highly integrated, the Fexe2x80x94Ni alloys is required to have improved punching property and enhanced strength for a thinner sheet.
With regard to the Fexe2x80x94Ni and Fexe2x80x94Nixe2x80x94Co alloys applied to the above mentioned uses, there have been proposed many means by adding Nb for improving etching property, press-deformability, strength and punching property. For example, JP-A-4-120251 proposes a shadow mask material in which an amount of soluble nitrogen (N) is reduced by limiting nitrogen to not more than 0.01%, by adding 0.01 to 1.0% of Nb which is susceptible to form nitrides, and further by adding a proper amount of Cr to improve adhesion property of a resist film and rigidity after press-deforming. JP-A-7-145451 proposes a lead frame material in which 1 to 4% of Nb is added to obtain high strength.
With regard to lead frame materials, JP-A-9-263891 proposes an Fexe2x80x94Ni alloy having high strength, low thermal expansion property and improved punching property, which comprises 0.003 to 0.03% of carbon, 0.005 to 2.5% of Nb, 0.001 to 0.02% of nitrogen and in which the alloy comprises niobium compounds (i.e. carbides and nitrides) having a particle size of not more than 20 xcexcm. JP-A-10-183304 proposes a method for improving press-deformability and mechanical strength of an Fexe2x80x94Ni alloy by controlling the size of particulate precipitates of elements, carbides, nitrides and/or intermetallic compounds which comprises elements of Group 4A or 5A to not more than 5 xcexcm, and by further controlling carbon and Nb amounts within ranges of 0.001 to 0.3% and 0.01 to 6%, respectively, to control precipitation of NbC (niobium carbide).
Further, JP-A-10-60528 proposes a high strength Invar alloy sheet for shadow masks and lead frames, which comprises up to 0.10% of C, up to 1.0% of Nb and not more than 0.005% of nitrogen.
The alloys or ideas mentioned above are effective means for improving various properties required to shadow masks or lead frames. However, in the case of the way wherein, with use of soluble Nb as a highly strengthening means for the alloy, a work strain is provided to the alloy in order to work-harden it, a lot of Nb is required, so that there arises a concern about deterioration of thermal expansion property. Further, in the case where the alloy is highly-strengthened with use of Nb-containing precipitates, which relies on the dispersion-strengthening effect primarily by niobium carbide, a lot of Nb is also required to ensure an enough amount of precipitates for attaining such an effect. It is also noted that the precipitates have a large size.
In the conventional alloys mentioned above, carbides and nitrides, which crystallize during solidification after ingot-molding, are liable to be coarse, causing problems that they may protrude from an etched surface and wear of a die is enhanced when press-punching.
Accordingly, an object of the invention is to provide an Fexe2x80x94Ni alloy having high strength and a small average thermal expansion coefficient, in which crystallization of coarse carbides and nitrides is restrained.
Another object of the invention is to provide a shadow mask made of the Fexe2x80x94Ni alloy and a Braun tube with the shadow mask.
Still another object of the invention is to provide a lead frame and a semiconductor element with the lead frame.
The present inventors tried to develop a new process for highly strengthening the Fexe2x80x94Ni alloy without relying on crystallized carbides which are liable to be coarse. As a result, they have finally found that the alloy can be refined to have fine crystal grains by precipitating, in the alloy structure, a lot of fine niobium compounds, which are primarily niobium nitrides and other compounds such as nitrides, carbides, and carbon nitrides, the niobium nitrides being capable to be super-finely precipitated by the solution/precipitation treatment, thereby the alloy can be highly-strengthened effectively without deterioration of various material properties. That is, the manner of improving the Fexe2x80x94Ni alloy is to precipitate super-fine niobium compounds with utilization of a small amount of additive Nb, more particularly, by adjusting each amount of Nb, carbon and nitrogen within an amount range entirely soluble in the Fexe2x80x94Ni alloy by the solution treatment. According to the improving manner, it is unnecessary to use a large amount of Nb, because it is intended to restrain crystallization of coarse carbides and nitrides and does not rely on the conventional method which is primarily based on dispersion strengthening.
Thus, according to a first aspect of the invention, there is provided an Fexe2x80x94Ni alloy having high strength and low thermal expansion, consisting essentially of, by mass percent, 30 to 50% of Ni, 0.005 to 0.1% of Nb, less than 0.01% of C, 0.002 to 0.02% of N, and the balance of Fe and inevitable impurities, wherein the following equation is fulfilled:
0.000013xe2x89xa6(% Nb)xc3x97(% N)xe2x89xa60.002.
According to a second aspect of the invention, there is provided an Fexe2x80x94Ni alloy having high strength and low thermal expansion, consisting essentially of, by mass percent, 27 to 47% of Ni, not more than 22% of Co, 0.005 to 0.1% of Nb, less than 0.01% of C, 0.002 to 0.02% of N, and the balance of Fe and inevitable impurities, wherein the following equation is fulfilled:
0.000013xe2x89xa6(% Nb)xc3x97(% N)xe2x89xa60.002.
In the Fexe2x80x94Ni alloy, preferably, the maximum grain size of compounds primarily comprising Nb and nitrogen and other compounds primarily comprising Nb and carbon is less than 0.5 xcexcm, which can be observed at an fractional section of metal structure, and a total number of the compounds is not less than 50,000/mm2 at an fractional section of metal structure.
Another preferable embodiment is that an average grain size of the metal structure of the Fexe2x80x94Ni alloy is not less than 10 by the crystal grain size number as defined in JIS G0551. The Fexe2x80x94Ni alloy may comprise not more than 0.10% of Mn and/or not more than 0.004% of B (boron).
According to a third aspect, the invention is directed to a shadow mask, which is made of the Fexe2x80x94Ni alloy and can be provided by working so as to have highly fine and precise form, and a Braun tube which has the shadow mask and an excellent color purity.
According to a fourth aspect, the invention is directed to a lead frame being made of the Fexe2x80x94Ni alloy and a semiconductor element which has the lead frame and can be highly integrated.